1. Field of the Invention
The present invention relates to a boundary acoustic wave device used for a resonator or a band-pass filter, for example. More particularly, the present invention relates to a boundary acoustic wave device using a boundary acoustic wave that propagates at the boundary between a piezoelectric layer and a dielectric layer.
2. Description of the Related Art
Conventionally, a surface acoustic wave device has been widely used for a band-pass filter of a mobile phone or the like. For example, Japanese Unexamined Patent Application Publication No. 2004-186868 discloses a surface acoustic wave filter. FIG. 14 illustrates the structure of the filter.
A surface acoustic wave filter 501 shown in FIG. 14 includes a piezoelectric layer 502 made of LiTaO3, and an IDT electrode 503 disposed on the piezoelectric layer 502. Sapphire 504 is attached to the back surface of the piezoelectric layer 502 opposite to a surface on which the IDT electrode 503 is disposed. Assuming that T is a thickness of the piezoelectric layer 502 and λ is a wavelength of a surface acoustic wave to be used, a ratio T/λ of the thickness of the piezoelectric layer 502 to the wavelength λ of the surface acoustic wave is at least 10. Accordingly, a spurious response is decreased, which is generated by the reflection of a bulk acoustic wave (BAW) at the boundary surface between the piezoelectric layer 502 and the sapphire 504.
Recently, a boundary acoustic wave device has been used instead of the surface acoustic wave device. The boundary acoustic wave device uses a boundary acoustic wave that propagates at the boundary between a piezoelectric layer and a dielectric layer. In the surface acoustic wave device, to excite the IDT electrode provided on the surface of the piezoelectric layer, a gap must be provided so as not to interrupt the vibration. Thus, the surface acoustic wave device has a relatively complicated and large package structure. In contrast, the boundary acoustic wave device uses the boundary acoustic wave that propagates at the boundary between the piezoelectric layer and the dielectric layer. Thus, a gap is not required, and the boundary acoustic wave device may have a relatively simple and small package.
Also, WO 2005/069486 discloses a boundary acoustic wave device having a layer structure including a sound absorbing medium, SiO2, an IDT electrode, and LiNbO3. Here, an acoustic velocity of a transverse wave at the sound absorbing medium is lower than an acoustic velocity of a transverse wave at SiO2. The sound absorbing medium is made of epoxy resin or porous SiO2. A high order mode spurious response is suppressed by providing the sound absorbing medium.
In addition, WO 98/52279 discloses a boundary acoustic wave device having a layer structure including a polycrystalline silicon film, a polycrystalline silicon oxide film, an IDT, and a single crystal piezoelectric substrate. Here, since the polycrystalline silicon film is provided, a boundary acoustic wave excited at the IDT electrode is enclosed in a portion before the polycrystalline silicon oxide film. The IDT electrode is reliably protected by the polycrystalline silicon film and the polycrystalline silicon oxide film.
Further, WO 2004/070946 discloses a boundary acoustic wave device in which an IDT electrode is arranged at the boundary between a piezoelectric layer and a dielectric layer and which uses a SH boundary acoustic wave. Here, the thickness of the IDT electrode is determined such that an acoustic velocity of the SH boundary wave is lower than acoustic velocities of transverse waves propagating at the dielectric layer and the piezoelectric layer. With this structure, an electromechanical coupling coefficient may be relatively large, a propagation loss and a power flow angle may be relatively small, and a temperature coefficient of frequency TCF may be set within a proper range.
In the boundary acoustic wave device, a higher order mode spurious response must be suppressed.
As described in Japanese Unexamined Patent Application Publication No. 2004-186868, the surface acoustic wave device can decrease the higher order mode spurious response as long as a solid, such as sapphire, is attached to the surface of the piezoelectric layer opposite to the surface on which the IDT electrode is disposed, and the ratio of the thickness of the piezoelectric layer to the wavelength of the surface wave is within a specific range.
Japanese Unexamined Patent Application Publication No. 2004-186868 discloses an example in paragraphs [0036] to [0042] in which an IDT electrode is provided on the front surface of 42° Y-cut X-propagation LiTaO3, and an R-plane cut sapphire substrate is laminated on the back surface of the LiTaO3. In this configuration, a first leaky surface wave primarily having an SH component propagates on the front surface of the LiTaO3 substrate. Paragraph (0034) of Japanese Unexamined Patent Application Publication No. 2004-186868 describes that, with the above-described structure, a spurious response may be generated by reflection of BAW at the boundary surface between the LiTaO3 and the sapphire substrate.
In contrast, based on the knowledge of the inventor of the present application, a generated spurious response is a high order propagation mode in which a P wave, a SH wave, and a SV wave propagate in a coupled manner. An acoustic velocity of the SV wave propagating in a propagation direction at the LiTaO3 with this cut direction is 3351 m/s, and an acoustic velocity of the SH wave is 4277 m/s. An acoustic velocity of a transverse wave at the sapphire is 6463 m/s. Since the acoustic velocity of the transverse wave at the sapphire is higher than the acoustic velocity of the SH wave at the LiTaO3, a high order mode spurious response primarily having a SH component and a high order mode spurious response primarily having a P+SV component are generated at the boundary between the sapphire and the LiTaO3. The 42° Y-cut X-propagation LiTaO3 substrate causes the SH wave to be strongly excited. Thus, a high order mode spurious response primarily having a SH component is strongly generated.
Japanese Unexamined Patent Application Publication No. 2004-186868 decreases the high order mode spurious response by increasing the thickness of the LiTaO3 substrate.
However, in Japanese Unexamined Patent Application Publication No. 2004-186868, when a frequency characteristic of FIG. 6 when the LiTaO3 substrate has a small thickness is compared with a frequency characteristic of FIG. 10 when the LiTaO3 substrate has a large thickness, a spurious response is suppressed to a greater extent in FIG. 10 of Japanese Unexamined Patent Application Publication No. 2004-186868 than in FIG. 6 of Japanese Unexamined Patent Application Publication No. 2004-186868. However, the number of appearances of spurious responses is increased. Namely, excitation intensity in a high order mode which may cause the spurious response is decreased by increasing the thickness of the LiTaO3 substrate. However, high order modes of many orders are included. Thus, a filter characteristic of the surface wave device is not sufficiently improved.
In WO 2005/069486, the sound absorbing medium with an acoustic velocity of a transverse wave lower than the acoustic velocity of the transverse wave at the SiO2 is provided on the SiO2. Accordingly, the high order mode can be decreased. However, a resin material, such as epoxy resin, polyimide, or liquid crystal polymer, which is used as the sound absorbing medium has a high linear thermal expansion coefficient. If the intensity of the structure including the SiO2, the IDT, and the LiNbO3 for propagation of the boundary acoustic wave is not sufficient, the structure may likely be expanded or contracted in accordance with the temperature of the boundary acoustic wave device on account of the effect applied by the linear thermal expansion coefficient of the sound absorbing medium. Thus, the absolute value of the temperature coefficient of frequency TCF of the boundary acoustic wave device may be increased.
With the boundary acoustic wave device disclosed in WO 98/52279, the piezoelectric single crystal substrate made of, for example, LiNbO3, is attached to Si with a small thermal expansion coefficient. Accordingly, the absolute value of the temperature coefficient of frequency TCF can be decreased. In this case, by providing SiO2, which is a low acoustic velocity material, is arranged between the Si and the LiNbO3 which are high acoustic velocity materials, a main propagation mode of the boundary acoustic wave is enclosed within the SiO2 region.
With the boundary acoustic wave device disclosed in WO 2004/070946, where metal, such as Au, that is heavier than Al is used instead of Al which is as light as SiO2, vibration energy in a main mode of the boundary acoustic wave does not spread over the entire SiO2, but is concentrated at the piezoelectric substrate. Thus, an electromechanical coupling coefficient K2 of the boundary acoustic wave can be increased.
However, with the structure in which the low acoustic velocity medium is arranged between the high acoustic velocity media such as the boundary acoustic wave device disclosed in WO 98/52279, the acoustic velocity may vary depending on the thickness and the film quality of the low acoustic velocity medium. As a result, a frequency variation may appear in the boundary acoustic wave device. Thus, precise quality control of the thickness and the film quality of the low acoustic velocity medium is required.
By increasing the thickness of the low acoustic velocity medium, the effect of variations in energy distribution in the main mode of the boundary acoustic wave due to variations in the thickness of the low acoustic velocity medium can be decreased. However, if the thickness of the low acoustic velocity medium is increased, a high order mode spurious response may be generated.
As described in WO 2005/069486, WO 98/52279, and WO 2004/070946, in the conventional boundary acoustic wave devices, various arrangements have been used to attempt to suppress the high order mode spurious response, to increase the electromechanical coupling coefficient, and to decrease the absolute value of the temperature coefficient of frequency TCF. However, unlike surface acoustic wave devices, with boundary acoustic wave devices, the high order mode spurious response is not sufficiently decreased.